1. Field of the Invention
This invention relates to a method of forming nitride films and in particular, but not exclusively, to the formation of silicon nitride films.
2. Description of the Related Art
The use of a thin film of various materials is ever expanding and in many applications it is becoming more common to provide stacks of films of different materials. Often such stacks require annealing, at some stage in the process, and this can result in blistering due to outgassing from lower layers and to cracking due to stress within the film.
One particular example of this occurs in the deposition of silicon nitride on a gallium arsenide substrate, for example as used in the production of GaAs field effect transistors, where dopant implantation of the GaAs substrate requires a subsequent high temperature anneal process to repair crystal lattice damage and “activate” the implant. This anneal, which is typically at 700° C. or more (although temperatures as low as 650° C. have been used), tends to cause out-diffusion of arsenic from the gallium arsenide substrate and thus requires a capping layer with good barrier properties and adhesion and crack resistance, especially at elevated temperatures. Typically silicon nitride is used. If the capping layer is to be simply sacrificial, it would typically have a thickness of less than 200 nm, but where it is going to have a functional property, e.g. as a dielectric layer, layers of 400 nm thickness or greater are known.
Current standard deposition processes result in blistering and, with the thicker films, cracking.